1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring the noise figure of the optical amplifier such as that used in linear repeaters in an optical transmission system formed by cascaded linear repeaters.
2. Description of the Background Art
A conventional linear repeater typically has a configuration as shown in FIG. 1A for a single stage amplification, or a configuration as shown in FIG. 1B for a two-stage amplification.
In the configuration of FIG. 1A, the linear repeater 30a comprises: an optical amplifier 31a formed by a rare earth doped optical fiber amplifier or a semiconductor laser amplifier for receiving and amplifying a signal light with a wavelength .lambda.s; and a narrow bandwidth optical filter 32 with a central transmission wavelength .lambda.c,r equal to the wavelength .lambda.s of the entered signal light, for removing amplified spontaneous emission (ASE) light generated at the optical amplifier 31a. In the configuration of FIG. 1B, the configuration of FIG. 1A is further equipped with an additional optical amplifier 31b, similar to the optical amplifier 31a, which is provided on an output side of the narrow bandwidth optical filter 32.
A conventional optical transmission system formed by cascaded linear repeaters typically has a configuration as shown in FIG. 2, in which the signal light outputted from the optical sender 41 is transmitted through a first optical fiber transmission line 42.sub.0 at which the signal light is attenuated at a loss L.sub.0 and entered into a first linear repeater 30.sub.1 at which the signal light is amplified at a gain G.sub.1 and then outputted to a next optical fiber transmission line 42.sub.1, and so on. Eventually, the transmitted signal light is received by an optical receiver 43 from the n-th linear repeater 30.sub.n through the n-th optical fiber transmission line 42.sub.n.
In this configuration of FIG. 2. each of the second and subsequent linear repeaters 30.sub.2 to 30.sub.n amplifies the ASE light propagated within the transmission bandwidth of the preceding narrow bandwidth optical filters as well, such that the optical intensity of the ASE light is sequentially accumulated. In the following, the ASE light generated at each linear repeater will be referred as the generated ASE light, while the ASE light entering into each linear repeater along with the signal light will be referred as the propagating ASE light, in order to distinguish them from each other.
A conventional noise figure monitoring apparatus for a linear repeater has a configuration as shown in FIG. 3, which generally comprises a gain detector 50 and an ASE optical power detector 60, both of which are connected with input and output sides of a linear repeater 30 such as that shown in FIG. 1A or FIG. 1B.
The gain detector 50 comprises: splitters 51a and 52a for splitting the lights entering into the linear repeater 30; an optical receiver 53a for detecting an optical power P.sub.s,in of the lights split by the splitter 52a; splitters 51b and 52b for splitting the lights outputted from the linear repeater 30; and an optical receiver 53b for detecting an optical power P.sub.s,out of the lights split by the splitter 52b.
An input side of the ASE optical power detector 60 comprises: the splitters 51a and 52a which are shared with the gain detector 50; a signal light polarization detector 61a for detecting a polarization state of the signal light split by the splitter 52a; a polarization controller 62a for converting the polarization state of the signal light into a linear polarization along a prescribed polarization direction; a driver 63a for setting the polarization direction for the signal light to be linearly polarized at the polarization controller 62a; a polarizer 64a for removing the linearly polarized signal light from an output of the polarization controller 62a; a narrow bandwidth optical filter 65a for extracting the ASE light from the output of the polarizer 64a; and an optical receiver 66a for detecting an optical power P.sub.ASE,in of the ASE light extracted at the narrow bandwidth optical filter 65a.
Similarly, the output side of the ASE optical power detector 60 comprises: the splitters 51b and 52b which are shared with the gain detector 50; signal light polarization detector 61b for detecting a polarization state of the signal light split by the splitter 52b; a polarization controller 62b for converting the polarization state of the signal light into a linear polarization along a prescribed polarization direction; a driver 63b for setting the polarization direction for the signal light to be linearly polarized at the polarization controller 62b; a polarizer 64b for removing the linearly polarized signal light from an output of the polarization controller 62b; a narrow bandwidth optical filter 65b for extracting the ASE light from the output of the polarizer 64b; and an optical receiver 66b for detecting an optical power P.sub.ASE,out of the ASE light extracted at the narrow bandwidth optical filter 65b.
Here, the outputs of the polarizers 64a and 64b are tile ASE lights having no specific polarization direction and the optical power reduced in half. Also, the central transmission wavelength .lambda.c,c used in the narrow bandwidth optical filters 65a and 65b is set equal to the wavelength .lambda.s of the input signal light.
The spectra of the signal light and the propagating ASE light at the point A on an input side of the splitter 51a in the configuration of FIG. 3 appear as shown in FIG. 4A, in which the propagating ASE light is confined within the transmission bandwidth of a narrow bandwidth optical filter of an immediately preceding linear repeater. The splitter 51a splits a small fraction of such entering lights while transmitting a majority of the entering lights to the linear repeater 30. The linear repeater 30 limits the bandwidth of the generated ASE light by a narrow bandwidth optical filter 32 provided therein to remove the generated ASE light outside of this transmission bandwidth. Here, the central transmission wavelength of the narrow bandwidth optical filter 32 is set equal to the wavelength .lambda.s of the entering signal light, so that the amplified signal light and propagating ASE light are transmitted through the linear repeater 30.
In a case the linear repeater 30 is of a single amplification type shown in FIG. 1A, the output of the linear repeater 30 contains the signal light and the propagating ASE light and generated ASE light which are within the transmission bandwidth of the narrow bandwidth optical filter 32 provided therein. In a case the linear repeater 30 is of a two-stage amplification type shown in FIG. 1B, the output of the linear repeater 30 is the output of the single amplification type linear repeater further amplified by the additional optical amplifier 31b.
The spectra of the signal light, the propagating ASE light, and the generated ASE light at the point D on an input side of the splitter 51b in the configuration of FIG. 3 appear as shown in FIG. 4B. The splitter 51b splits a small fraction of such entering lights while transmitting a majority of the entering lights to the optical fiber transmission line.
The lights split at the splitter 51a are further split into two at the splitter 52a and one of the split lights is received at the optical receiver 53a. The optical power of the received light at the optical receiver 53a is that of the signal light reached to the optical receiver 53a through the splitters 51a and 52a as can be seen in FIG. 4A. Therefore, the input signal optical power P.sub.s,in at an input terminal of the linear repeater 30 can be obtained by adding the signal light loss from the point A to the optical receiver 53a to the optical power of the received light at the optical receiver 53a.
Similarly, the lights split at the splitter 51b are further split into two at the splitter 52b and one of the split lights is received at the optical receiver 53b. The optical power of the received light at the optical receiver 53b is that of the signal light reached to the optical receiver 53b through the splitters 51b and 52b as can be seen in FIG. 4B. Therefore, the output signal optical power P.sub.s,out at an output terminal of the linear repeater 30 can be obtained by adding the signal light loss from the point D to the optical receiver 53b to the optical power of the received light at the optical receiver 53b.
On the other hand, the other one of the lights split at the splitter 52a is entered into the signal light polarization detector 61a at which the polarization state of the signal light is detected, and then entered into the polarization controller 62a at which it is converted into the linearly polarized light, along the polarization direction set by the driver 63a which is to be removed by the polarizer 64a. Then, at the polarizer 64a, the signal light is almost completely removed, while the optical power of the propagating ASE light having no specific polarization direction is reduced in half. Then, at the narrow bandwidth optical filter 65a, the output of the polarizer 64a within the transmission bandwidth .DELTA..nu. is extracted, and received by the optical receiver 66a. The optical power of the received light at the optical receiver 66a is that of the propagating ASE light transmitted through the splitter 51a to the narrow bandwidth optical filter 65a. Consequently, the propagating ASE optical power P.sub.ASE,in at the input terminal of the linear repeater 30 can be obtained by adding the transmission loss from the point A to the optical receiver 66a to the optical power of the received light at the optical receiver 66a.
Similarly, the other one of the lights split at the splitter 52b is entered into the signal light polarization detector 61b at which the polarization state of the signal light is detected, and then entered into the polarization controller 62b at which it is converted into the linearly polarized light, along the polarization direction set by the driver 63b which is to be removed by the polarizer 64b. Then, at the polarizer 64b, the signal light is almost completely removed, while the optical power of the propagating ASE light having no specific polarization direction is reduced in half. Then, at the narrow bandwidth optical filter 65b, the output of the polarizer 64b within the transmission bandwidth .DELTA..nu. is extracted, and received by the optical receiver 66b. The spectra of the signal light, the propagating ASE light, and the generated ASE light at the point E on an input side of the optical receiver 66b in the configuration of FIG. 3 appear as shown in FIG. 4C, such that the optical power of the received light at the optical receiver 66b is that of the propagating ASE light and the generated ASE light reached to the optical receiver 66b through the splitter 51b to the narrow bandwidth optical filter 65b. Consequently, a total ASE optical power P.sub.ASE,tot at the output terminal of the linear repeater 30 can be obtained by adding the transmission loss from the point D to the optical receiver 66b to the optical power of the received light at the optical receiver 66b.
Here, for the sake of the simplicity, the signal light excess loss due to the other optical components such as the splitters provided on the transmission line or the optical isolators omitted in FIG. 3 is assumed to be zero.
Now, the noise figure F of the linear repeater 30 can be expressed by the following equation (1): EQU F=P.sub.ASE /(h.multidot..nu..multidot..DELTA..nu..multidot.G)+1/G (1)
where h is the Planck constant, .nu. is an average frequency of the ASE light, G is a gain of the linear repeater 30, and P.sub.ASE is the optical power of the generated ASE light within the bandwidth .DELTA..nu. which is generated by the linear repeater 30.
Here, the gain G of the linear repeater 30 can be obtained from the input signal optical power P.sub.s,in at the input terminal of the linear repeater 30 that can be determined from the optical power of the received light at the optical receiver 53a, and the output signal optical power P.sub.s,out at the output terminal of the linear repeater 30 that can be determined from the optical power of the received light at the optical receiver 53b, according to the following equation (2). EQU G=P.sub.s,out /P.sub.s,in ( 2)
Also, the generated ASE optical power P.sub.ASE can be expressed in terms of the propagating ASE optical power P.sub.ASE,prop within the bandwidth .DELTA..nu. at the output terminal of the linear repeater 30 and the total ASE optical power P.sub.ASE,tot within the bandwidth .DELTA..nu. at the output terminal of the linear repeater 30 that can be determined from the optical power of the received light at the optical receiver 66b, by the following equation (3). EQU P.sub.ASE =P.sub.ASE,tot -P.sub.ASE,prop ( 3)
Here, the propagating ASE optical power P.sub.ASE,prop within the bandwidth .DELTA..nu. at the output terminal of the linear repeater 30 cannot be detected by itself, but it can be obtained from the gain G of the linear repeater 30 and the propagating ASE optical power P.sub.ASE,in within the bandwidth .DELTA..nu. at the input terminal of the linear repeater 30 that can be determined from the optical power of the received light at the optical receiver 66a, according to the following equation (4). EQU P.sub.ASE,prop =G.multidot.P.sub.ASE,in ( 4)
Thus, by substituting the propagating ASE optical power P.sub.ASE,prop obtained from this equation (4) into the above equation (3), the generated ASE optical power P.sub.ASE of the linear repeater 30 can be calculated, and by substituting the gain G and the generated ASE optical power P.sub.ASE obtained from the above equations (2) and (3) into the above equation (1), the noise figure F of the linear repeater 30 can be calculated.
In other words, the conventional noise figure monitoring apparatus has a configuration in which the input signal optical power P.sub.s,in, the output signal optical power P.sub.s,out, the propagating ASE optical power P.sub.ASE,in, and the total ASE optical power P.sub.ASE,tot are measured by using the gain detector 50 and the ASE optical power detector 60, and the noise figure of the linear repeater 30 is calculated from the gain G of the linear repeater 30 and the generated ASE optical power P.sub.ASE determined from these measured quantities.
Consequently, such a conventional noise figure monitoring apparatus requires to provide the gain detector 50 and the ASE optical power detector 60 on both of the input side and the output side of the linear repeater 30, and this requirement in turn causes the size and the cost of the noise figure monitoring apparatus to be considerably large.
Now, in an optical amplifier called the erbium doped fiber amplifier (EDFA) for realizing the amplification by pumping the erbium doped fiber with the laser or others, in general, the majority of the noise energy obtained at the output terminal can be considered as due to the beat noise between the shot noise of the entered signal light and the ASE light entered along with the signal light.
An example of a conventional noise figure monitoring apparatus for such an EDFA has a configuration as shown in FIG. 5. In this configuration of FIG. 5, an output of a laser diode 91 of a distributed feedback type which functions as a signal light source is transmitted to the first stage optical amplifier (EDFA) 93 through a transmission fiber 92.sub.1, and an output of this first stage optical amplifier 93 is transmitted through the band-pass optical filter (BPF) 94 and a transmission fiber 92.sub.2 to a second stage optical amplifier 95 whose noise figure is to be monitored and whose output is connected to a noise figure measurement unit 96.
In this second stage optical amplifier 95, the input terminal is connected with a fiber coupler 98 through an optical isolator 97, while the output terminal is connected with the fiber coupler 98 through an erbium doped fiber 99. Here, the erbium doped fiber 99 is formed by doping the aluminum compound Al.sub.2 O.sub.3 in addition to the erbium. The fiber coupler 98 has one of its inputs optically coupled with a laser diode 100, while the erbium doped fiber 99 is optically coupled with the noise figure measurement unit 96 through the output terminal.
The noise figure measurement unit 96 has a photo-diode 102.sub.1 which is optically coupled with the erbium doped fiber 99 of the second stage optical amplifier 95 through an optical filter (OF) 101 on one hand and electrically connected with one input of a noise figure calculation unit 103. The output terminal of the erbium doped fiber 99 of the second stage optical amplifier 95 is connected with a fiber monitor 105 through an optical isolator 104, and one output of the fiber monitor 105 is connected with an output terminal through a band-pass optical filter (BPF) 106.sub.1 while another output of the fiber monitor 105 is optically coupled with a photo-diode 102.sub.2 through a band-pass optical filter (BPF) 106.sub.2 which is electrically connected with another input of the noise figure calculation unit 103.
Here, each of the band-pass optical filters 106.sub.1 and 106.sub.2 is formed by the vapor deposition of the dielectric multi-layer on a glass plate, while each of the photo-diodes 102.sub.1 and 102.sub.2 is formed by InGaAs semiconductor suitable for a long wavelength bandwidth.
In the noise figure calculation unit 103, the outputs of the photo-diodes 102.sub.1 and 102.sub.2 are A/D converted by A/D converters 107.sub.1 and 107.sub.2 and entered into a digital processing circuit 108, while an output of the digital processing circuit 108 is D/A converted by D/A converter 109 and outputted as an approximated noise figure F*.sub.pre to be described in detail below.
In this noise figure monitoring apparatus of FIG. 5, the signal light of a prescribed wavelength .lambda.s=1.5515 .mu.m outputted from the laser diode 91 is attenuated through the transmission fiber 92.sub.1, and given to the optical amplifier 93 at a prescribed level of -15 dBm. The optical amplifier 93 then amplifies the received signal light at the prescribed gain of 24.9 dB. The band-pass optical filter 94 has a central wavelength equal to the wavelength .lambda.s of the signal light, and its transmission half bandwidth is set to 2.7 nm which is appropriate for transmitting a major energy of the signal light, such that optical amplifier 93 reduces the ASE light component from the amplified signal received from the optical amplifier 93 in order to suppress the beat noise between the ASE light and the signal light and then outputs the resulting signal to the optical amplifier 95 through the transmission fiber 92.sub.2.
In the optical amplifier 95, the signal light given through the optical isolator 97 is combined with as pumping light of a prescribed wavelength 0.98 .mu.m outputted from the laser diode 100 at the fiber coupler 98 and transmitted to the erbium doped fiber 99 which functions as a gain medium, along with the propagating ASE light component also given through the optical isolator 94, as shown in FIG. 6A. Here, the optical power on the vertical axis is normalized by the signal optical power obtained at an output of the erbium doped fiber 99.
The erbium doped fiber 99 amplifies the signal light by being pumped by the pumping light, and outputs the amplified signal light and propagating ASE light along with the generated ASE light which is newly generated at the erbium doped fiber 99 itself, as shown in FIG. 6B.
In the noise figure measurement unit 96, the optical filter 101 has a high wavelength range transmission characteristic, so as to remove the component for the wavelength 0.98 .mu.m of the pumping light scattered from the spontaneous emission light which is outputted from the erbium doped fiber 99.
Then, the photo-diode 102.sub.1 applies the photo-electric conversion to the ASE light obtained by the optical filter 101 in this manner, and supplies the obtained electrical signal corresponding to the optical power of the ASE light into the noise figure calculation unit 103.
On the other hand, the input terminal of the optical isolator 104 receives the signal light, propagating ASE light, and generated ASE light from the erbium doped fiber 99 as shown in FIG. 6B, along with the above described pumping light. Here, however, a part of the components of the pumping light are already absorbed at the erbium doped fiber 99, and in addition, almost the entire components of the pumping light are absorbed at the optical isolator 104. Consequently, the fiber monitor 105 splits the signal light and the propagating and generated ASE lights at a prescribed ratio of 20:1.
The optical filter 106.sub.1 has a bandwidth transmission characteristic as shown in FIG. 7A in which the central transmission wavelength is set equal to the wavelength .lambda.s=1.5515 .mu.m of the signal light, so as to suppress the propagating and generated ASE light components and transmit the signal light in its output.
The optical filter 106.sub.2 has a bandwidth transmission characteristic as shown in FIG. 7B in which the central transmission wavelength is set equal to a wavelength .lambda.c2=1.542 .mu.m&lt;.lambda.s in a vicinity of the wavelength .lambda.s of the signal light, with the transmission half bandwidth equal to 1.5 nm, so as to suppress the signal light and transmit the generated ASE light components in its output, as shown in FIG. 6C. Here, the propagating ASE light is ignored as it is smaller than the generated ASE light as much as about 40 dB in this transmission bandwidth of the optical filter 106.sub.2 as shown in FIG. 6B.
The photo-diode 102.sub.2 applies the photo-electric conversion to the optical power P.sub.ASE (.lambda.c2) of the generated ASE light obtained by the optical filter 106.sub.2 in this manner, and supplies the obtained electrical signal corresponding to the optical power of the generated ASE light into the noise figure calculation unit 103.
In the noise figure calculation unit 103, the optical power of the ASE light obtained through the photo-diode 102.sub.1 is A/D converted by the A/D converter 107.sub.1 and supplied into the digital processing circuit 108. The digital processing circuit 108 then calculates the gain G&gt;&gt;1 of the erbium doped fiber 99 by using the known procedures such as those disclosed in Japanese Patent Application Laid Open No. 4-356984 (1992).
Now, in general, the noise figure F is given by the equation (1) described above, but the approximated expression of the following equation (5) holds for the generated ASE optical power for a case of the wavelength .lambda.c2 in a vicinity of the wavelength .lambda.s, by using a predetermined constant C&lt;0. EQU P.sub.ASE (.lambda.s).apprxeq.P.sub.ASE (.lambda.c2).multidot.10.sup.(-c/10) ( 5)
When this approximated expression is substituted into the above equation (1) and the logarithm of both sides is taken, the following expression (6) for the approximated noise figure F* in dB unit can be obtained. ##EQU1## where K.sub.1 .tbd.10.sup.(-c/10) /h.multidot..nu..multidot..DELTA..nu., and the value of P.sub.ASE (.lambda.c2) is obtained by A/D converting the output of the photo-diode 102.sub.2 at the A/D converter 107.sub.2.
The approximated noise figure F*.sub.pre in dB unit so calculated is plotted as a curve C1 in FIG. 8, which has an error .DELTA.P with respect to the theoretical value of the noise figure F* in dB unit obtained according to the above equation (1) and plotted as a curve C2 in FIG. 8, due to the saturation characteristic of the gain G characteristic to the erbium doped fiber 99 and the level variation of the generated ASE light with respect to the variation of the gain G associated with that saturation characteristic, where this error .DELTA.P varies according to the input signal optical power.
From the above equations (1) and (6), this error .DELTA.P can be expressed by the following equation (7) with the ASE powers in dB unit P.sub.ASE *(.lambda.c2) and P.sub.ASE *(.lambda.s). EQU .DELTA.P=F*.sub.pre -F*=P.sub.ASE *(.lambda.c2)-P.sub.ASE *(.lambda.s) (7)
which varies according to the gain G* in dB unit as plotted as a curve C3 in FIG. 9.
Now, in such a conventional noise figure monitoring apparatus for the erbium doped fiber amplifier, despite of the fact that the calculations carried out in the noise figure calculation unit 103 are quite complicated as in the above equation (6), the error .DELTA.P contained in the approximated noise figure F*.sub.pre given by the above equation (7) has been regarded as a constant value C as indicated in FIG. 9 according to the approximation of the above equation (5), such that the dependency of the approximated noise figure F*.sub.pre on the gain G* as indicated in FIG. 9 has effectively been ignored. As a result, there has been cases in which the approximated noise figure F*.sub.pre takes quite large value in accordance with the variation of the input signal optical power.
Such a variation of the error .DELTA.P with respect to the variation of the gain G* can be reduced by setting the central wavelength .lambda.c2 of the transmission bandwidth of the optical filter 106.sub.2 to be very close to the wavelength .lambda.s of the signal light. However, this provision has been very often impractical due to the technological and economical limitations, as it requires to set up the selective transmittivity of the optical filters 106.sub.1 and 106.sub.2 extremely sharply in order to realize the level difference greater than a prescribed lower limit between the signal light and the generated ASE light at the outputs of these optical filters 106.sub.1 and 106.sub.2.